<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>
    <div ng-app="myApp" ng-controller="myCtrl">
        firstname <input type="text" ng-model="first"><br> lastname <input type="text" ng-model="last"><br>
        <h2>{{last+ " " +first}}</h2>
        fullname
        <h1>{{fullname}}</h1>
    </div>
    <script src="http://cdn.static.runoob.com/libs/angular.js/1.4.6/angular.min.js"></script>
    <script>
        var app = angular.module('myApp', []);
        app.controller('myCtrl', function($scope) {
            $scope.last = "";
            $scope.first = "";
            $scope.$watch('last', function() {
                $scope.fullname = $scope.last + " " + $scope.first;
            })

            $scope.$watch('first', function() {
                $scope.fullname = $scope.last + " " + $scope.first;
            })
        })
    </script>
</body>

</html>